Radio warning system



Jan. 16, i945.

G. L. BEERS RADIO WARNING SYSTEM Filed Feb. 27, 1942 2 Sheets-Sheet 1 INVENTOR Gfof ff/Ps. BY 7/45 ATTORNEY Jan. l 16, 1945. G. l.. BEERS RADIO WARNING SYSTEM Filed Feb. `27, 1942 2 sheetssheet 2 lNvENToR so/aps Z. i2-5ms.

BY WK ATTORNEY Patented Jan. v1e,l 1945 y RADIO WARNING SYSTEM George L. Beers, Haddonield, N. J., assigner to Radio Corporation of America, a corporation ofv Delaware Y Application February` 27, 1942, 'Serial No. 432,607 f Claims. (Cl. Z50-2) My present invention relates to improved radio warning systems, and more particularly to a receiverA of the sol-called alert type which vis responsive to a radiated control signal -which appearsv as a sub-audible tone modulationV on a radiated carrier wave. Y Y l In the past there have been disclosed various .types of radio warning'v systems. One of these systems, as disclosed' byA H. fB. Deal in application dSerial No. 403,736, filed' July 23, 1941, patented mittedcarrier-with one sub-audible tone for a few seconds would make theloudspeaker operative, while modulation fora fewseconds with another sub-#audible tone would silence the loudspeaker. The purpose of that system was to assure reception of emergency information, as in air raid warnings, making use of existing broadcast stations Without interference at other times with the program, and without requiring listening to all the programs. In that system provision was, also, 'made for operation of a visual, or audible, alarm signal.

Another system of the radio warning type has beenv disclosed by W. R.'Koch in application Serial No. 404,902, illed` July 3 1, 1941, patented'March '21, 1944, as S. Patent No. 2,344,618.' In that quiring only 'a single sub-audible control tone transmitted during the alert period.

system electrical tuned circuits are employedfor the selection of the sub-audible control tones instead of tuned-reed relays. These electronic tone selectors essentially comprise resistance- 'capacity networks employing regenerative feedback for improving the selectivity characteristic of the system, and inverse, or degenerative, feed-y backfor stabilizing theamplier operation Without utilizing a high degree of degenerative feedback. In this way there was secured low-cost electrical toneselection, in addition to simplification of the-entire' system. Specifically, the

receiver of the aforesaid Koch application employed a i'lrst'tube operating as a. rst detectorlocal oscillator, a second tube operating as an intermediate frequency amplifier,l a third tube' .of my present invention to provide a receiver rof lthe type referred to above, `but which isimproved Another important object of the present invention is to provide an air raid warning system utilizing only a single control modulation tone thereby reducing the selectivity necessary in the tone selecting` circuit, since it is only necessary to provide suicient selectivity to separate the control tone from the, normal modulation. A `single tuned circuit of relatively poor Q is, therefore, suiiicient in the tone-selector circuit.

' Another object of my invention is to provide an alert type of receiver designed to provide an alarm signal consisting of aloud, interrupted audible tone, and the tone being produced whenever .the subfaudible modulation tone is transmitted; the audible tone continuing until the user operates a switch to put the receiver in a listen position, y

Anotherfobject of the invention is to provide a radio warning receiverv which can be muted, without the expense of a power relay, by blocking oi the audio amplifier through the use of a superaudible oscillator; a warning signal being provided by permitting the audio amplifier of the receiver to oscillate whenever the sub-audible modulation tone is being transmitted.

Still other objects of this invention are to imvprove generally the simplicity and eiciency of a receiver of the alert type, and more especially to reduce the cost and weight of such receivers.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, asfto both its organization and `method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. A

In thedrawings: A Y Y Fg.- 1 shows in schematic form the essential networks of my present system;

Fig.- 2 shows the circuit details of the invention. `A Referring now to the accompanying drawings, the present systemis show n in block diagram 'form in Fig. l. It will be seen that generally the system comprises a signal collector device l, which vmay be of any well known and desired type. As explained previously the collector I will in variousvaspects, and the receiving Asystem recollect the usual broadcast carrier Waves in the broadcast band of 550to1700 kilocycles (kc.). The collected signals are transmitted to a con- Ventional first detectorlocal oscillator network f broadcast transmitter.

2 thereby to reduce the collected modulated carrier waves to an intermediate frequency (I. F.).` For example, an I. l'l. oi the order of 455 kc. may be employed, as is weil known in broadcast reception practice. The network 3 is an I. F. amplifier, and the ampliiied I. F. energy is fed to a second detector 4. There is derived from the detected modulated carrier energy the modulation voltage of the loud speaker will be the pro-gram modulation which was imposed upon the carrier at the However, when the receiver is switched to the alert condition and the control tone is transmitted, then the output of the loud speaker will be the audible warning signal. When the control tone is not transmitted and the receiver is in the "alert condition, there will be no sound output from the loud speaker.

To provide the audible warning, there is connected a feedback path 8 between the power output stage 6 and the audio ampliiier 5. This feedback path provides an audio oscillation network which can be adjusted to pro-vide a distinctive,

. hereto-fore, 'this selector may be of the electronic type The output of the selecto-r 9 is fed to a rectifier I0, and the rectified voltage is utilized to control the operation of a super-sonic, or superaudible, oscillator H.

The use of the super-sonic oscillator I I provides two distinct advantages. In the lrst place, it can be started and stopped by applying a proper control potential and thus provide a definite onoff characteristic. The second advantage is that the rectiiied` output of the oscillator provides a source of high negative potential for controlling the output stage, which otherwise would not be available without materially increasing the cost of the +B supply system.

The super-audible oscillations are rectified in a rectifier circuit I2, and the rectied voltage is 'employed to control the effectiveness of the power output stage 6. The feedback for producing the alarm tene is effective only when the receiver is in the alert condition and the bias on' the output stage is normal, i. e., when the super-sonic oscillator II is rendered inoperative. In turn, the super-sonic oscillator can operate only when the receiver has been switched to the alert condition so as to be prepared to select from the modulated carrier the sub-audible modulation tone, and when no such tone is being transmitted'. Obviously in order to condition the receiving, system so as to receive the usual programs in the broadcast band, or in the particular band which the receiver is operating-in, it is only necessary to stop the super-audible oscillator and open the subaudible control path and the feedback path. The receiver operation band may be in the short wave range, the frequency modulation range or the televisio-n range. It will now be seen that the receiver is muted by blocking oiT' the audio amplifier through the use of a super-audible oscillator I I which is adapted to be rendered inoperative in ilo response to the sub-audible modulation tone, and a warning signal being provided by permitting the feedbackI path 8 to produce audio oscillations whenever the alert tone is being transmitted.

Referring, now, to the specific circuits of the receiving system, which circuits are shown in Fig. 2, it is first pointed out that the receiving system comprises tube 2 whichv includes thev electrodes for the combined first detector--local oscillatornetwork. This tube may be of the 12SA7 type. The I. F. amplier tube 3 may be of the 125137 type. The tube 4' may be of the 12SQ'7 type, and, as shown, performs simultaneously the functions of demodulation, audio frequency amplication and automatic volume control (AVC) bias production. The power output tube 6' can be of the 35L6GT type, and the loudspeaker, of course, can be of any Well known and desired type.

The tube 9 is included in the sub-audible tone selector network, and can be of the 12'SJ7 type. The rectier IB, shown in Fig. 1, is provided by the auxiliary diode included in tube 4. The super-audible oscillator includes the tube II which may b e of the 12SA7 type as in the case of tube 2', and rectier I2' may be included in the tube 3. It is to be clearlyunderstood that these tube types are purely' illustrative, since any other types of tubes which can. perform the functions of the tubes shown herein can be employed.

There will rst be described the well known vbroadcast receiving circuits o f the receiving system. The signal collector I may be a grounded antenna circuit, 'and is shown as including a coupling winding I3. This winding may be coupled to the loop I4 provided inthe signal input circuit of the tube 2'. Since the various circuits of the rst 4detector--local oscillator are rpurel)7 conventional it is believed sucient to. make brief reference thereto. The variable condenser I5 is connected in shunt with the loop I4, and is capable of providing a tunable input circuit which can be adjustedover the entire receiving range. The tank circuit of the local oscillator section of tube 2 is designated -by numeral I6, and variable condenser I6 adjusts the tank circuit over a range of oscillation frequencies which differs from the signal input range by the frequency of the I. F. output circuit I1.

Those skilled in the art are fully aware of the fact that the primary and secondary circuits II and I'I of the I. F. input transformer I8 are each fixedly tuned to the operating I. F. Value. Of course, this value may be chosen from a wide range of frequency values. The I. F. amplifier 3' has its cathode atgroundpotential; the cathode of tube 2 is shown connected to an intermediate tap on the coil of tank circuit I6. The resonant output circuit I9 of I. F, amplifier tube 3 is fixedly tuned to the operating I. F. value. Circuit I9 is reactively coupled to the resonant input circuit 20 of the demodulator, or second detector, circuit The demodulator is providedv by a diode whose anode 2I is located adjacent to the common cathode o1" multiple function tube 4. The load resist-or 22 is connected between the low potential side of circuit 2i! and the grounded cathode of tube c. The direct-current voltage component of the rectified I. current developed across resistor 22 is utilized for AVC (automatic volume control) by application to the signal grids of tubes 2 and' 3". Thus, the anode end of resistor 22 is connected 'by' the AVC lead 23 to thelcw potential-sides of input circuit III-'I5 and input circuit Il. The filter resistorA 23 is included in switch 42.

components inthe-AVC bias. f v.

, 'In' shunt with theiresistor22 are connected the resistor 24 and Vcondenser 25 which providea path l adapted t feed the sub-audiblecontrol tone voltage to the adjustable contact of switch'26 when the latter is closed. The resistor 21. is connected from the junction of resistor 24 andconde-nser 25 to the xed contact of switch'26. Theswitch 26 is shown'in the alert position. The triode section of tube 4 comprises the common cathode, a'control grid, and a plate 28. Theplate 28 is coupled' tol the controlgrid of .the power output tube. 6. VThe control V grid of the audioainplier section of tube 4' is connected by audio coupling condenser 29 and lead 30 to the adjustable contact of switch 26.

The amplifier tube 6 has its cathode connected to lground through a self-biasing resistor, .The

signal'input gridis connected to ground through a path comprising resistor 3|, resistor 32,and resistor 33. The junction o'f resistors 3|v and32 is ley-passed to ground by an audio by-pass condenser. The audio coupling condenser 34 trans# mits the amplified audio frequency voltage output of the audio amplifier section of tube 4 to the signal input grid of tube 6'. The resistor 35 is arranged in circuit with plate 28, and is connected to apoint -of proper positive potential on the power supply system of the receiver. The r'e'- maining electrodes of tube 6 are energized from the power supply system in theusual manner as shown, and the' audio output transformer v 4H couples the plate circuit of tube 6 to the voice coil of loudspeaker The alarm feedback path for providing the audio oscillations tov produce the alarm tone coniprises the condenser "4| connected between the plate of vtube 6 and the adjustable contacty of The switch 42 is shown adjusted to feed audio signal voltage from the plate ,of tube 5 to lthecondenser 2.9 connected to the signal input grid of the audio amplifier section of tube 4. Of course, when the switch 42 is adjusted to the'free, or L, contact of the switch then feedback is not had. It vwill be understood that the positions of switches 2E, w42. andl 64, shown in Fig. 2, are the alert, or A,v positions of these switches. When switch' 26 Vis adjusted to the ffL contact then condenser 29 is connected tothe adjustable tapr5|l slidable along the load resistor 22. This means that audio voltageA is fed to theAv audio amplier from the loadresistor, and that program modulation voltage is derived thereacross In'the position of switch 26 shownin-Fig. 2vthe` network 24'25-2`| acts as a filter network to attenuate all modulation frequencies, except the subaudilble modulation frequency which* can be as low as 24 cycles.

The powerrectifier 5I, ofl the' diode type, may be utilized to provide the power supply lvoltage f-orthe power output tube f6' `and' the other tubes of the system "as shownin Fig. 2. The lead 52 is shown connected to the 'cathode of diode 5|,the cathode being connected to ground through a proper electrolytic condenser. f The anodelof diode' 5| is connected to one line of a power supply audible frequency. The 'selection' ofthe particular frequency to .be used depends upon the wishes oi'the'designer. For example, the circuit 6| may b e tuned to a frequency of 20 kc. The third grid, or oscillator grid, is vcoupled as at 62 to the tuned circuit 6|. f

' lIt'lwill'be seen that the oscillator is oi a well known form. lThe second and fourth grids of tube 'l l are connected to ac t as a positive shielding grid, and are connected througha resistor and lead 63 to the A contact of switch'4. The adjustable contact of switch G4 is connected to a point of proper positive potential on the power supply, circuit. The L, contact of rswitch 64 is free, and, therefore, when switch 64 is in that position the screen grids of tube Il' are de-energized and the oscillator will not function. I-Ience,l in the listen position'of switch 64 the supervaudible oscillator will not be functioning.

The super-audible oscillationsproduced are utilized to providea muting, or cut-ofhbias for the signal grid of tube 6'. This is provided by connecting the anode of diodel I2. to the ungrounded end of resistor 33. That same end of resistor 33 0"'. In other words, the resistor 33 acts as a load resistor for the diode circuit which includes .the

electrodes `ofvdiode l2. The rectified voltage developed'across resistor 33 is impressed upon the signal grid of tube 6',and biases the power output tube suiliciently to .be `ineffective to transmit any audio voltage to the reproducen 'As pointedout inFig. 1, the oscillator is under. the control of the sub-audible modulation toneon the'received carrier. Referring again to Fig. 2, it will be noted that the first grid, ad-A jacent the grounded cathode, of tube'll is connected through a path comprising resistor 4||l, lead 1| andlead '|2 to the second auxiliary anode of tube 4'. This auxiliary anode is designated by the numeral I0' to indicate that it is the anode of the diode which provides the rectifier |0 schematically shown in Fig.11. The diode I0 recties the sub-audible modulation tone which is transmitted through the selector network. The rectier l0 develops a bias voltage for biasing tube` |I' to cut-off thereby eliminating the source of the muting bias for tube 6.

Considering'the circuit of the sub-audible tone Y selector, it ispointed out that the selector consists of a Anetwork constructed in a manner disclosed generally in the aforesaid Koch application. The 'cathodev of tube 9' is connected to ground through an unby-passed resistor 8 0. The controlgrid 8| isrconnected by lead 82 to the condenser 34 througha pathincluding coupling condenser 83, resistor `81| and lead 85. The plate 86 of tube 9' is connected to a point of proper positive 'potential of the' power supply network plug 53 which includes in circuit therewithv a n control switch 54. Since itis not desired to have any reproduction from the speaker whilev the receiver is in the alert"condition,'unless a con-y trol 'signal is" ori the received carrien-there'is provided the super-audible .oscillator tubey I.r This tube has its plate' IIJ".` connected toa resonant' circuitJSl.. whichifis'tuned4 toa' desired super'-l through resistor B'|.1 The screen grid of tube 9 is similarly connected, through a properly bypassed reducing resistor, to the plateener'gizing circuit. It will be understood thatlead` 85 acts as the input lead` whichfeeds the control tone voltage from the audio system to the tone selec-` tor circuit.' The path 86-82 acts as a regenerative feedback path for the sub-audible modulaprises a; plurality of series connected condensers anda plurality of shunty resistors.

It is believed sufiicient for the purposes of this applicationk to point out that the Rf-C network of the selector circuit provides a simple method of securing selectivity at the sub-'audible modulation frequency. It is desired to have the alert circuits function when the percentage of control tone modulation of a received carrierwave is or greater. In the present case the selector circuit uses only Rf-C elements with positive and negative feed-back to procure stable selectivity. In order to secure frequency selective positive feedback a phase shift of4 180 degrees is necessary at the desired frequency,iand as much difference from this value as can be readily secured at all other frequencies. Networks having zero degrees phase shift at the desired frequency can be used, but a second amplifier tube must be employed to secure the 180 degrees phase shaft. From a cost and stability standpoint a single tube is preferable, and, therefore, a network furnishing 180 degrees phase shift was used.

While three-section networks can be made: to give 180 degrees phase shift, four sections will give it with less attenuation, and, therefore, greater positive feedback.v This, in turn, permits greater stabilizing negative feedback, and, therefore, more stable operation. More sections would give 180 degrees phase shift at more than one frequency. The series C-shunt R type was adopted, because it iittedin with voltage supplyv to tube 9'. It,l also, `provided ground potential for direct cur'- rent at points in the circuit to which input and output could be coupled. A resistor can be inserted in series with grid 8|' to suppress any overload or response to high frequencies;

The tuning of the R-fC circuit can be easily adjusted by changing or shunting one or more of the resistors, and the Q brought to the desired value by adjusting the magnitude of resistor 8'0. The effective Q of the selector circuit at the sub-audible modulation frequency of 24 cycles is approximately l0. The output terminal of the tone selector circuit is the junction of the first two series condensers adjacent plate 86, and the junction is connected by lead |00 to lead-` 12. In other words, the selector circuit output is fed to the rectifier l0', and the rectified voltage developed is employed, through lead 1|, to bias the rst grid of. tube I l' to cut-olf.

Summing up, then, the various characteristics and functions of the receiving system disclosed herein, it will be first noted that only a single sub-audible control tone is employed. The use of only a single control tone reduces the selec-V tivity necessary in the tone selectingcircuit, and, hence, a single tuned circuit of relatively poor Q is found to be suiiicient. 'The receiver is designed to provide an alarm signal consisting of a loud, interrupted audible tone. This tone is producedv whenever the sub-audible modulation is applied to the: received carrier, and. continues until the user throws the simultaneously controlled switches 26, 42 and 64 into the listen position.

The muting of the reproduction occurs when the switches 26, 42 and 64 are setin the alert position and no control tone is' being received. This causes the super-audible oscillator to be effective, and biases off power output tube 6. When the sub-audible modulation tone is present on the carrier the former is selected by the tone selector; the selected voltage` being rectified` so as to bias the oscillator Il' to cut-off thereby permitting output tube 6 to function. 0f course,

the feedback path 'andthe'audio circuit are .also functioning. Therefore, there will be emitted from the loudspeaker a loud audible tone. Upon the oscillator Il" being rendered ineffective, due to the rectified modulation, developed across resistor 88', the bias of tube 6' will be normal, because no rectified voltage will be developed across resistor 33. I

It will be understood that when the switches are all adjusted for alert the alarm tone emitted from the reproducer, in response to existence of the sub-audible modulation tone on the carrier, is intermittent. due to the blocking action of the rst audio grid circuit. In order to hear instructions during the alert or alarm period, the listener will throw switches 26, 42 and 64 into the L position. The alarm signal will cease, and the'operation of the receiver is then normal. Upon conclusion of the instructions the switches may then be returned to the alert position, and the receiver will be muted until the broadcasting station again transmits the subaudible modulation tone. The energizing connections from the power supply rectifier to the various electrodes of the system are denoted by numeral 200. The cathode heater elements will, of course, be continuously energized.

r'IIhereceiving system shown herein sounds an alarm tone, permits instant reception, has low first cost, low stand-by and operating power consumption. Moreover, it is reliable and of light weight, and is little affected by temperature variations andcan bemanufactured in quantities without alteration of existing facilities. course, if several services are required, each using itsrespective sub-audible tone, vibrating reeds, or cascaded electrical circuits, are necessary. It will be understood that in place of the Rf-C network shown in the tone selector circuit, mechanical resonators of any desired or well known type capable of producing an output voltage for biasing 01T the super-audible oscillator, may be employed. For example, there may be used mechanical resonators driven by Rochelle salt crystals. In some cases asecond crystal can be used to secure electrical output from the resonator instead of contacts. With such arrangements it is easy to secure selectivity corresponding to coupled circuits. Lumped constants for the mechanical resonators Will be necessary to prevent response toharmonics of the fundamental frequency.

While I have indicated and described a system for carrying my invention into eifect, it will be apparent to lone skilled in the art that -my invention is by no `means limited to the particular organization shown and described, but that many modications may be made without departing from the scope of my invention, as set forth in the appended claims.

-What I claim is:

pl. In a radio receiving system including a sourceof sustain-edl audio frequency oscillations, means for producing super-audible oscillations, means for rendering said source of audio frequency oscillations inoperative in response to said super-audible oscillations being produced, and means, responsive to reception of a predetermined signal, for rendering said super-audible oscillation producing means ineffective.

2. A radiov receiving system including, in combination,an audio network constructed to act as a source of sustained audio frequency oscillations, means for producing super-audible oscillations, nmeans for rendering said. source ofaudio -received modulated carrier wave, means vfor pro- -frequencylfoscillations inoperative in response to u viding audio frequency oscillations, .meansl for re- "said super-audible oscillations, and a selective lmeans, resonanty to the predetermined. frequency ;ofr a received'signal, for rendering said super- `audible oscillation meansineffective.. `3. A' radio receivingY system including, in coml bination, .means for.' producing sustainedl audio frequency oscillations, a source of super-audible oscillations'.means,tincluding said super-audible source, for preventing the production of said audio frequency oscillations, and signal-responsive means for rendering said super-audible oscillation source inoperative.

4. A radio receiving system including, in combination, means for producing audio oscillations, a source of super-audible oscillations, means, including said super-audible source, for Preventing the production of said audio oscillations, and additional means, responsive to a received signal, for rendering said super-audible source inoperative thereby operating said audio oscillation means to produce an alarm.

5. In a radio warning receiving system including an audio frequency amplifier, provided with a regenerative feedback circuit to produce Warning-representative oscillations, means including a super-audible oscillator for supplying cut-off biasing potential to said audio frequency ampliner, and signal-responsive means for rendering inoperative said super-audible oscillator whereby said warning-representative oscillations are produced by said audioampliiier.

6. In a radio receiving system including, in combination, means responsive to a sub-audible control tone modulation on a received carrier Wave for producing a. control potential, a source of audio frequency oscillations, and means, in-

cluding a source of super-audible oscillations and a rectifier, for utilizing the control potential derived from said first named means to control said source of audio frequency oscillations.

'7. In a radio receiving system including, in combination, a source of audio frequency oscillations and a loudspeaker responsive thereto for producing an alarm signal, means normally preventing operation of said source, means responsive to a sub-audible control tone modulation on a received carrier wave for 4deriving a control potential, and means for utilizing said control potential to render said first means ineffective thereby to cause said source of audio frequency oscillations and loudspeaker to produce an alarm signal whenever the received carrier wave is modulated with said sub-audible control tone.

8. In a radio receiving system, an audio frequency amplifier and loudspeakel` for reproducing an alarm signal, means, including said audio frequency amplifier, for producing audio frequency oscillations for providing said alarm signal, means responsive to a sub-audible control tone modulation on a received carrier wave for deriving a control potential, means for utilizing said control potential to cause said means for producing audio frequency oscillations to produce an alarm signal whenever the received' carrier Wave is modulated with said sub-audible control n tone, and additionalmeans constructed and arranged to render said source of audio frequency oscillations inoperative but permit said audio frequency amplier and loudspeaker to reproduce the audio frequency modulation on the received carrier Wave.

9. In combination with a source of carrier Wave energy whose modulation includes a control tone, means for deriving control tone voltage from a producing such oscillations, means `for1pre`venting rendering said device ineifective thereby to per-A the received carrier Wave,and additional means,

`responsive to the presence of saidvr modulation control, tone von the saidcarrier Wave,for rendering said last named means ineffective.

10.;A methodof radio .reception which includes collecting a carrier wave modulated with a sub-audible tone, deriving from the collected Wave modulation voltage `of said tone, producing oscillations of an audio frequency, reproducing the oscillations to producean alarm, preventing the production of said audible oscillations in the absence of said tone on the collected Wave, and automatically rendering said oscillation production effective in responseito the derivation of said modulation tone Voltage.

11. A method of producing an audible alarm upon the reception of a carrier Wave having modulation which includes a control frequency, comprising deriving from the received modulated carrier wave voltage of said control frequency, producing audio oscillations to provide an audible alarm, rendering said oscillation lproduction ineffective in the absence of said modulation control frequency on the received wave, and automatically rendering said oscillation production effective with said control frequency voltage.

12. In a radio Warning system of the type employing at least a carrier amplifier, means to apply to the amplifier a carrier wave modulated with a Warning signal, a demodulator to derive warning signal voltage, an audio amplier, a regenerative feedback circuit electrically associated With the audio amplifier to produce oscillations in the audible range, means for reproducing the oscillations, a device for normally preventing production of the oscillations and means, responsive to said warning signal voltage, for

mit said audio frequency oscillations to be produced.

13. In a radio Warning system of the type einploying at least a carrier amplifier, means to apply to the amplifier a carrier Wave modulated with a Warning signal, a demodulator to produce warning signal voltage, an audio amplier, a'

regenerative feedback circuit electrically associated with the warning voltage amplifier to produce oscillations in the audible range, means to provide super-audible oscillations, means for de- .in the absence ofsaid ,modulation component,

and means selectively responsive to said subaudible frequency for rendering said source ineffective in response to said modulation component being included in the modulation of said carrier wave.

15. In a radio receiver of the type comprising responsive to the l.existenc'ze of a. modulation control .frequency in the .modulation of said carrier waves, means vresponsive 'to the output of said -selector circuit for rendering `said super-'audible oscillator ineffective, and additional means for selectively and concurrently rendering .said feedback circuit and super-audible oscillator inoperative.

i `GrIilORGE L. BEERS 

